Vinyl chloride resins are widely used as soft resin. However, it is known that the vinyl chloride resins generate harmful substances upon burning. For this reason, it has been strongly required to develop a substitute resin. Recently, there have been used, as a substitute resin, olefin polymers produced by using a metallocene catalyst. Example of these olefin polymers include a copolymer of ethylene and α-olefin. However, such a copolymer poses a problem that it becomes too tacky when softened. Further, shaped articles, such as film, produced from the copolymer have poor surface properties because of deteriorated transparency, rigidity and low-temperature heat sealability, thereby rendering it unsatisfactory. The copolymer is also poor in processability.
Crystalline propylene polymer films are widely used as a packaging film owing to their excellent rigidity, transparency and moisture resistance. The crystalline propylene polymer films may be used alone as monolayered films, or in combination with films of ethylene-vinyl acetate copolymer, linear polyethylene or the like as multilayered laminate films. For their use, the packaging films are often processed into bags, regardless of whether the films are monolayered or multilayered. A series of processes for forming a film into a bag and closing the opening of the bag after putting contents therein is generally carried out by pressing the film edges with a heated rod and melt-welding the edges to each other (so called “heat sealing”). Recently, it has been required to speed up the bag-forming and packaging processes for increased productivity, which results in great demands for materials with good heat sealability. Also, it has been required for the film to inevitably have slipping and anti-blocking properties so that the above fabrication process can be efficiently made.
However, films of propylene homopolymers are defective in that, when heat sealed, the films need to be pressed at high temperature for a long time. To overcome such a drawback, there has been commonly used a technique of copolymerizing propylene with ethylene, 1-butene or any other α-olefin. However, in the conventional technique, it is necessary to copolymerize propylene with a large amount of comonomer including ethylene, 1-butene or other α-olefin so as to obtain copolymers having satisfactory heat sealability. Further, these comonomers often remain dominantly in the low-molecular fractions to form a tacky low-crystalline component that makes the copolymers tacky (such a component is hereinafter referred to as “tacky component”). As a result, films of the copolymers fails to show a good rigidity that is intrinsic to polypropylene films, and tend to cause blocking between films, thereby impeding the fabrication process. In addition, the whitening of films due to bleeding occurs to deteriorate the appearance of films. For these reasons, the copolymers are not applicable to practical use. Heretofore, it has been attempted to remove the tacky component by dissolving them in an inert solvent. It is however difficult to wash out the tacky component efficiently while preventing the low-temperature melting component that contribute to heat sealability from being lost by the washing, this making the washing method unsatisfactory for industrial use.
Recently, the film forming process has come to be conduced by using a large film forming machine to speed up the process for increased productivity, this changing the film-forming conditions. The quality of films needs to be maintained regardless the change in the film-forming conditions. On the other hand, there is a problem that multilayered laminates fails to show sufficient interlaminar strength when produced without using an adhesive and the like. It is therefore necessary to adhere films by using an adhesive in order to impart excellent properties of low-temperature heat sealability, rigidity, transparency, moisture resistance and the like to the multilayered laminates.
Thus, a first object of the present invention is to provide a polypropylene film that is less tacky, well-balanced between a tensile modulus and a heat seal temperature, and excellent in processability, transparency and impact resistance. A second object of the present invention is to provide a polypropylene film that is less tacky and excellent in resistance to whitening due to peeling, softness and impact resistance. A third object of the present invention is to provide a multilayered polyolefin resin laminate having an excellent low-temperature heat sealability and exhibiting a sufficient interlaminar strength without using an adhesive or the like.